126 MAMMALIA. 



pass forwards to meet corresponding horizontal fibres proceeding from the more 

 anterior tracts of the oblong medulla. By this arrangement the thicker portion of the 

 restiform body participates in the motive functions of the voluntary tracts, whilst the 

 external layer, continuous with the floor of the fourth ventricle, is only superimposed ; 

 any sensitive properties, therefore, which the restiform body imparts to the auditory 

 nerve, are solely derived from this external continuation with the sensitive centre. 



The preceding descriptions have been made by examining the brain from the 

 summit and base, but for pointing out more plainly the origins of the nerves, it is 

 necessary to begin at the median aspect in a separate hemisphere divided into three 

 longitudinal or peripheric regions : the median, the intercedent, and the external. 



The median region is connected with that of the opposite hemisphere by the great 

 commissure, for the purposes of the intellect ; in many examples of the lower kind of 

 mammalia it is extremely limited, especially when convolutions cease to appear, so 

 that the surface of the brain is nearly even. 



The intercedent region is placed between the median and external, and on 

 removing the superficial layer of grey matter of the striated body and the epithelium 

 of the ventricle, the fibres appear which pass to their respective convolutions. Two of 

 the convolutions communicate by tracts with the motive segments of the crus of the 

 brain ; one, forming the true visual, is inserted into the anterior angle of the thalamus 

 and mammillary body ; one is conveyed to the involuntary centre, in the oblong 

 medulla ; the others, in the posterior part, are conveyed in tracts, which spread out 

 in numerous fibres on the posterior portion of the lateral ventricle and on the 

 descending horn, for communicating with the thalamus, the optic tract and the sensitive 

 centre formed by the ventricular cord. The same arrangement exists in simiae, the 

 horse and ox, whilst in man there are four larger convolutions in the place of the 

 two in these animals anterior to the true visual, without mentioning the origin of 

 the olfactory nerve. 



The external or motive region occupies the convolutions on the outer side of the 

 intercedent, and its tracts pass as the lower bed of fibres in the grey matter connected 

 with the striated body. It consists of three layers, the first or inner layer occupies a 



